In most mining and quarrying applications, the floors of truck bodies are constructed of flat plates welded into the structure.
The floor plates are welded to the sides of the body and to supporting beams on the underside of the floor plates. The floor plates are generally made from high strength abrasion resistant steels.
Mining truck bodies are typically very large. Payload capacities in excess of 100 tonnes are common and in the largest trucks, payloads are greater than 300 tonnes. During truck loading operations, loads up to about 100 tonnes may be dropped several meters directly onto the floor of the truck body.
The material loaded into mining or quarrying trucks may vary widely in nature, even in the one mine. In some applications it may be mostly large, hard, sharp cornered and very abrasive rocks. In another application the payload material may consist of smaller and softer rocks that are very abrasive. In yet another application, the payload may have a high proportion of cohesive material that sticks to parts of the body and does not shed fully from the body during load tipping operations. A mining truck body and particularly the floor must be able to handle wide variations in rock impact, abrasive wear (mainly during the load tipping/dumping operations) and cohesiveness of the material carried.
Mining trucks are typically expected to have a working life of at least 60,000 operating hours and during this time a single truck could experience about 300,000 load-haul-dump cycles.
The thickness of the steel truck body floors are typically in the range of 16 to 50 mm.
Thicknesses greater than about 25 mm are typically made up of a base plate and a high hardness wear resistant steel plate welded on top of the base plate. The top plate may be selectively placed rather than uniform over the whole area of the floor. Sometimes, spaced apart bars are used to reduce abrasive wear of the floor plate. Furthermore, numerous large supporting beams are required under these floor plates. These beams are required to prevent excessive bulging type permanent deformation of the floor when large rocks are dropped onto it.
Replacement or substantial repair of the truck body floor is typically required at least twice during the operating life of a mining truck. This repair work generally necessitates exchange of the truck body with a new or repaired body or that the truck spends a lengthy time in a workshop. The repair of truck body floors is a significant cost item for many mining trucks.
In an effort to overcome the problems and costs associated with floors made from flat steel plates, the use of suspended rubber floors in truck bodies has also become established in the mining industry. In this case, the floor consists of a single thick piece of rubber supported by numerous cables spanning between beams at the base of the side sections of the body. The cables are made of multiple strands of steel or elastomeric material.
The cables act to carry the vertical forces from the load in the body via tension in the cables similar to the way the cables of a suspension bridge carry the loads from the road section of a “suspension bridge”.
The main advantages of the suspended rubber floor are:                Moist clay containing cohesive materials are less likely to stick to the body when it is tipped to dump the load.        When worn out or badly damaged, the floor can be replaced relatively quickly.        The empty weight of the body is sometimes less than for an all steel body of equivalent capacity.The improved shedding of cohesive (sticky) materials mainly results from the flexing of the rubber floor during load tipping operations.        
The disadvantages of the suspended rubber floor are:                The initial purchase cost is higher than for an all steel truck body.        Frequent re-adjustment of the floor support cables is required (to adjust for permanent stretching that occurs).        Intermittent and un-predictable replacement of failed or severely damaged cables is required.        Replacement floors are expensive.        
Because of the above difficulties, the use of suspended rubber floors has been limited to less than 10% of all mining applications. Their use is mainly restricted to applications where the improved shedding of sticky materials is very important and/or where the reduction of truck empty weight is particularly critical.
Analytical modelling work and mine site trials have shown that an alternative to the above-described floors, namely suspended curved metal (typically steel and hereinafter described in that context) truck body floors, can be effective in at least the mining industry.
In any given application, a curved steel plate for a floor is rolled with a single plane of curvature. The curved steel floor plate is then supported only at the two sides of a truck body so that it curves down from the supporting points at the sides.
The curved steel floor plate provides the general load containing function and acts as a tension member to transfer the vertical forces from the load on the floor to tension forces which are transferred into beams at the base of the sides of the body. Because the curved steel floor plate carries the forces arising from the payload primarily through tension forces within the plate, it is sometimes referred to as a steel membrane floor. However, in practice the stiffness of the plate (arising from the need to provide a long life against abrasive wear), the high variability in the placement of the loads carried, the use of a single radius of curvature rather than a parabolic curvature, and eccentricity of the load transfer points on the edges of the floor, means that the curved steel floor plate is also subjected to moderate bending loads. Unless it is severely overloaded, the curved steel floor plate experiences only small changes from its initial shape. This type of floor is hereinafter described as an edge-supported curved steel plate floor.
FIG. 1 shows the principle of the edge-supported curved steel plate floor. The floor plate 11 shown in the Figure is rolled to a constant radius R. The radius need not be constant, but a constant radius of curvature provides satisfactory performance and is easier to manufacture than any other curved shape. The floor plate 11 is supported at the side edges 12 of the plate. The edge forces F that are generated by the plate 11 act tangentially to the edges of the floor plate. Because of the curvature of the floor plate, the tangent line is at an angle (θ) above the horizontal. The vertical component of the tangential forces (F×sin θ) balances the weight of the floor plate plus the payload carried by the floor plate.
The edge-supported curved steel plate floor provides the potential for:                A lower empty truck weight without increasing the manufacturing cost for the truck.        Rapid, low cost replacement of the truck body floor.        Improved shedding of cohesive (sticky) materials compared to the conventional rigid all steel bodies.        
Compared to the suspended rubber floor, an edge-supported curved steel plate floor better absorbs concentrated impact loads within itself. Consequently, impact induced concentrations in the forces within the supporting system at the sides of the floor plate are much lower than for the supporting cables of a suspended rubber floor system.
Several edge-supported curved steel plate floors have performed successfully in extended mine site trials during 1996 and 1997. These floor systems were for a large rear dump mining truck with a rated payload capacity of approximately 180 tonnes.
The general cross sectional configuration and the edge connection configuration of curved steel plate floors used in the trials are shown in FIGS. 2 and 3.
The radius of curvature of the floor plates was approximately 1.1 times the width between the edge supports.
FIG. 2 shows a generalised cross-section through a truck body with an edge-supported curved steel plate floor 11.
With reference to FIG. 2, the body includes an array of transverse beams 29 and longitudinal beams 30 that are welded together and a pair of opposed box section side beams 23 that are supported by the transverse beams 29. The longitudinal beams 30 maintain the spacing between the central portions of the transverse beams 29. They also transfer the loads from the body to the chassis of the truck. The body also includes upstanding sides 28 extending from the side beams 23.
FIG. 3 shows the attachment system for the curved steel plate floor 11 shown in FIG. 2 in larger detail.
With reference to FIG. 3, the floor attachment system includes an abutment block 21 welded to the floor plate 11. The abutment block 21 bears against a round bar 22 that is attached to the box section side beam 23 of the body via a formed support plate 24 and brackets 25. The floor plate 11 is further retained (in the downwards direction) by retainer blocks 26. These retainer blocks are also welded to the side beam 23. The retainer blocks 26 also provide support for corner plates 27 which attach to the sides 28 of the body and prevent payload material from passing into and through the floor attachment system.
In most applications and particularly in mining truck applications, it is desirable to have the largest practical radius of curvature for the floor plate so that the centre of gravity for the payload is as low as possible because increasing the height of the centre of gravity for the payload reduces the stability of the truck and increases the stresses on many of the truck components during cornering, braking etc.
Table 1 set out below illustrates how the radius of curvature of an edge-supported curved metal plate floor affects the height of the payload centre of gravity.
TABLE 1Effects of Changing Radius of Curvature of Floor PlatesIncreased height ofF/(Wp +Payload Centre ofR/WΘWf)h/wGravity1.030.0° 1.000.134C + 0.043 w1.127.04°1.100.120C + 0.039 w1.227.62°1.200.109C + 0.036 w1.322.62°1.300.100C + 0.033 w1.420.93°1.400.092C + 0.030 w1.519.47°1.500.086C + 0.029 wThe height change shown in Table 1 is referenced to that for a flat plate floor positioned on top of straight transverse beams having the same depth as the transverse beams for the frame that supports the edge-supported curved steel plate floor. In the design used for the above mentioned mine site trials, the clearance “C” corresponded to approximately 0.03 W. This clearance is required to allow for substantial elastic deflection of the floor plate that can occur under severe localised loading impacts, for example when a very large rock is dropped from a height of several meters directly onto the floor plate.
Table 1 also shows how the radius of curvature for the curved metal plate floor affects the mean edge supporting force. Higher edge supporting forces mean higher stresses in the floor plate, higher loads in the edge attachment system, higher loads in the longitudinal beams along the sides of the body, and higher loads in the transverse beams under the floor plate.
The design radius of curvature of the curved steel floor plate is a trade-off between the payload centre of gravity height and forces in the floor plate, the attachment system and the supporting structure. It may be possible to increase the radius of curvature beyond 1.1 W when more experience is gained with this type of floor system. With this experience, it may also be possible to reduce the clearance “C” below what has been used to date.
The shape of the curved steel floor plate varies from the initial static unloaded condition depending on the load it carries. Besides the changes in loading that occur for the static truck condition, other changes occur during loading (when large localised dynamic impact loads can occur), during travel of the truck over uneven ground and during tipping of the load. These changes of loading in the floor plate and the shape of the floor plate make it desirable that the connections between the floor plate and the beams at the base of the side sections of the body are “hinged” joints. If the curved steel plate floor was rigidly attached to the beams at the base of the side sections of the body, this would prevent the steel plate from functioning as a flexible member and also it would cause strongly varying bending stresses at this joint. If for example the attachment was some form of welded connection, besides destroying the desired flexibility in the floor plate, the varying bending stresses in the welded joint would be likely to cause fatigue failures in the welded joint. A welded joint would also make replacement of the floor plate much more difficult, much more time consuming and significantly more expensive.
The attachment system shown in FIGS. 2 and 3 was found to be effective in the above-mentioned trials. The offset between the contact zone on the abutment attached to the floor plate and the centre-line of the floor plate creates a bending moment in the floor plate. For the geometry of FIG. 3, the peak magnitude of the resulting bending stresses are about 10 times the magnitude of the average tensile stress arising from the tension in the floor plate that is generated by the payload plus self weight of the curved steel floor plate. Provided the abutment attached to the floor plate makes reasonably uniform contact with the round bar, this is not a problem because the average tension generated tensile stresses in the floor plate are generally low, for example about 15 Mpa, and the floor plate is made of hard wear resistant steel with a tensile strength generally in the range of 1200 to 1600 Mpa. The average tensile stresses at the sides of the floor plate are low because the plate must be thick enough to provide good dent resistance against impacts from large rocks during the truck loading process and to provide for wear that occurs during load tipping operations. The wear is generally greatest near the central rear sections of the floor plate, not in the region of the edge connections.
However, this attachment system suffers from the cost of constructing the supports on the side beam at the base of the sides of the body and the difficulty of aligning the round bar on the beam with the bar on the floor plate during manufacture. High stress concentrations and subsequent failure problems can occur if the contact between the two bars is strongly irregular.
In mining applications, truck bodies are subject to extreme loading conditions that can cause the floor plate and/or the supporting beams to become distorted. If this happens, the load transfer from the edge-supported curved steel floor plate to the side beams becomes concentrated at some locations rather than uniformly distributed over the full length of the support. With the attachment system of FIGS. 2 and 3, it is difficult to remedy this situation and failure of the floor attachment system can occur. Further, when replacement of the edge-supported curved steel floor plate is required, it is difficult to match the position of the round bar on the side beams to the position of the abutment bar on the new floor plate. This difficulty can cause extra costs or short life of the floor attachment system after replacement of the floor.
The feasibility of an edge-supported curved steel plate floor was demonstrated in the above-mentioned mining truck trials during 1996 and 1997. However, to date this type of truck body floor has not been commercially adopted for mining truck or other applications. The main reasons for non-adoption of this technology are:                The cost of manufacturing the attachment system between the floor plate and the frame of the body.        The difficulty of achieving a good uniform contact between the abutment bar on the floor plate and the mating round bar on the frame when replacement of the floor plate is required.        Uncertainty about the ability of this floor to shed sticky materials.        